1. Field of the Invention
This invention relates to a process for making an oxidative-coloration layer (a layer capable of forming color on oxidation) which is a component of an all solid-state electrochromic device.
2. Description of the Prior Art
The so-called electrochromic phenomenon is a change in a visible-ray absorption spectrum of a material, said change accompanying a reversible electrochemical reaction (oxidation-reduction reaction) caused by the application of electric signals. The display element utilizing this phenomenon is referred to as an electrochromic device (abbreviated as ECD). Attempts have long been made to develop a solid-state ECD and apply it to a superimposition indicator or aperture of a camera. A solid-state ECD comprising a cell of the structure: electrode/WO.sub.3 /V.sub.2 O.sub.5.Cr.sub.2 O.sub.3 /Cr.sub.2 O.sub.3 electrode reportedly has a life of 5.times.10.sup.6 cycles or more (Japan, J. appl. phys., 19,2121(1980) and ibid., 20, 575(1981)).
However, the Cr.sub.2 O.sub.3 layer used in the above cell is disadvantageous in that the layer itself is slightly colored even in the bleached state and therefore the light transmittance of the cell as a whole is lowered. A cell free from this drawback has been proposed, wherein an iridium hydroxide or nickel hydroxide layer which is transparent in the bleached state is employed in lieu of the Cr.sub.2 O.sub.5 layer (U.S. Pat. No. 4,350,414).
The above iridium hydroxide and nickel hydroxide layers are formed by the reactive ion plating method. However, the formation of these hydroxides by this method involves knotty problems, and additionally the ECD thus prepared is insufficient in electrical responsiveness. Hence, practical use of these hydroxides are questionable.